1. Field of the Invention
The present invention generally relates to a thermal control system for an electrochemical fuel cell, as well as a method for improving thermal conditions for cold start of an insulated fuel cell stack.
2. Description of the Related Art
Electrochemical fuel cells convert reactants, namely fuel and oxidant fluid streams, to generate electric power and reaction products. Electrochemical fuel cells employ an electrolyte disposed between two electrodes, namely a cathode and an anode. The electrodes each comprise an electrocatalyst disposed at the interface between the electrolyte and the electrodes to induce the desired electrochemical reactions. The location of the electrocatalyst generally defines the electrochemically active area.
Polymer electrolyte membrane (PEM) fuel cells generally employ a membrane electrode assembly (MEA) consisting of an ion-exchange membrane disposed between two electrode layers comprising porous, electrically conductive sheet material as fluid diffusion layers, such as carbon fiber paper or carbon cloth. In a typical MEA, the electrode layers provide structural support to the ion-exchange membrane, which is typically thin and flexible. The membrane is ion conductive (typically proton conductive), and also acts as a barrier for isolating the reactant streams from each other. Another function of the membrane is to act as an electrical insulator between the two electrode layers. The electrodes should be electrically insulated from each other to prevent short-circuiting. A typical commercial PEM is a sulfonated perfluorocarbon membrane sold by E.I. Du Pont de Nemours and Company under the trade designation NAFION®.
The MEA contains an electrocatalyst, typically comprising finely comminuted platinum particles disposed in a layer at each membrane/electrode layer interface, to induce the desired electrochemical reaction. The electrodes are electrically coupled to provide a path for conducting electrons between the electrodes through an external load.
In a fuel cell stack, the MEA is typically interposed between two separator plates that are substantially impermeable to the reactant fluid streams. The plates act as current collectors and provide support for the electrodes. To control the distribution of the reactant fluid streams to the electrochemically active area, the surfaces of the plates that face the MEA may have open-faced channels formed therein. Such channels define a flow field area that generally corresponds to the adjacent electrochemically active area. Such separator plates, which have reactant channels formed therein are commonly known as flow field plates. In a fuel cell stack, a plurality of fuel cells are connected together, typically in series, to increase the overall output power of the assembly. In such an arrangement, one side of a given plate may serve as an anode plate for one cell and the other side of the plate may serve as the cathode plate for the adjacent cell. In this arrangement, the plates may be referred to as bipolar plates.
The fuel fluid stream that is supplied to the anode typically comprises hydrogen. For example, the fuel fluid stream may be a gas such as substantially pure hydrogen or a reformate stream containing hydrogen. Alternatively, a liquid fuel stream such as aqueous methanol may be used. The oxidant fluid stream, which is supplied to the cathode, typically comprises oxygen, such as substantially pure oxygen, or a dilute oxygen stream such as air. In a fuel cell stack, the reactant streams are typically supplied and exhausted by respective supply and exhaust manifolds. Manifold ports are provided to fluidly connect the manifolds to the flow field area and electrodes. Manifolds and corresponding ports may also be provided for circulating a coolant fluid through interior passages within the stack to absorb heat generated by the exothermic fuel cell reactions. The preferred operating temperature range for PEM fuel cells is typically 50° C. to 120° C., most typically between 60° C. and 85° C.
Under typical conditions, start-up of the electrochemical fuel cell stack is at a temperature above the freezing temperature of water, and the fuel cell stack can be started in a reasonable amount of time and quickly brought to the preferred operating temperature. In some fuel cell applications, it may be necessary or desirable to commence operation of an electrochemical fuel cell stack when the stack temperature is below the freezing temperature of water (commonly referred to as “freeze-start” conditions), and even at subfreezing temperatures below −25° C. However, at such low temperatures, the fuel cell stack does not operate well and rapid start-up of the fuel cell stack is more difficult. It may thus take a considerable amount of time and/or energy to bring an electrochemical fuel cell stack from a starting temperature below the freezing temperature of water up to an efficient operating temperature.
A variety of techniques have been developed and/or proposed to address this issue, including the addition of various heating elements and/or heat-exchanging subsystems that are designed to quickly increase the temperature of the fuel cell stack. Another technique involves insulation of the fuel cell stack itself. Thus, if the ambient temperature is at or below the freezing temperature of water, the stack temperature may stay above freezing for some extended period of time following shut down, which permits more favorable starting conditions should the stack be restarted during this period of time.
While advances have been made associated with cold start of fuel cell stacks, there remains a need in the art for improved and/or more efficient techniques relating to the same. The present invention fulfills such needs and provides further related advantages.